The long-term objectives of the proposal are to elucidate the molecular mechanisms that drive the pathogenesis of glioblastoma, the most common and aggressive primary brain tumor. We are taking a novel neurodevelopmental perspective to the study of glioblastoma pathogenesis. Glioblastoma tumors are thought to arise from the transformation of astrocytes or their precursor cells, the neural stem cells. During brain development, the transcription factor STAT3 plays a critical role in the regulation of neural stem cell fate specification including their differentiation into astrocytes. The central hypothesis of the project is that deregulation of STAT3 signaling contributes to glioblastoma pathogenesis. Using a rigorous mouse genetics approach, we have discovered that STAT3 plays opposing oncogenic and tumor suppressive roles in astrocytes depending on the mutational profile of the tumor. The major genetic alterations of deficiency of the tumor suppressor PTEN and expression of the oncogenic protein EGFRvIII may mark distinct sets of glioblastoma tumors. Remarkably, we have found that STAT3 suppresses cell transformation downstream of PTEN deficiency, whereas STAT3 behaves in an oncogenic manner downstream of EGFRvIII in astrocytes. We have also found that STAT3 represses IL8 transcription in PTEN-deficient glioblastoma cells and thereby inhibits their proliferation and invasiveness. Our findings raise fundamental questions on STAT3's role and mechanisms in glial malignancy. What is the mechanism by which STAT3 mediates the ability of EGFRvIII to induce astrocyte transformation? How does STAT3 repress IL8 transcription in PTEN-deficient glioblastoma cells? What is the role of STAT3 signaling in the biology of human glioblastoma cancer stem cells? We propose to address these questions by achieving the following three specific aims, (1) determine the mechanism by which STAT3 mediates EGFRvIII-induced astrocyte transformation by identifying the genes that operate downstream of STAT3 in this pathobiological response, (2) identify the transcriptional regulators that couple the STAT3 signal to IL8 repression in glioblastoma cells, and (3) determine the function of STAT3 in the malignant potential of glioblastoma cancer stem cells in vitro and in vivo. The proposed experiments represent an important set of experiments that will significantly improve our understanding of the transcriptional mechanisms that govern glioblastoma pathogenesis. The proposed studies should also lay the foundation for potential identification of novel therapeutic strategies in patient-tailored treatment of glioblastoma.